Electron tube seal structure



Oct. 2, 1951 W. W. EITEL ETAL ELECTRON TUBE SEAL STRUCTURE Filed May 31,1950 INVENTORS WILL/AM M E/TEL MAQT/N E. Wan-'5 Patented Oct. 2, 1951UNITED STATES PATENT OFFICE Burlingame, Calif.,

assignors to Eitel-Mc- Cullough, Inc., San Bruno, Calil'., a corporationof California Application May 31, 1950, Serial No. 165,334

3 Claims.

Our invention relates to electron tubes, and more particularly to theglass-to-metal seals at the lead-in conductors of such tubes.

In vacuum tubes having a glass wall through which a metal lead issealed, there is a serious problem of glass cracking at the seal.Frequently these cracks do not develop until after a tube has been inoperation for a period of time, but once a, crack occurs the tube is nolonger vacuum tight and is therefore inoperable, which accounts i'ormany tube failures in radio transmitters and the like.

Th usual practice in tube making is to form a glass-to-metal seal at thelead-in conductor which may be a tungsten rod, the glass used being onehaving a coefficient of expansion matching that of the metal lead.Ordinarily the metal lead is first oxidized since a metallic oxide layerproduces a better bond between the glass and metal. In the case ofenvelopes made of preformed glass blanks, the practice is to seal aglass head to the lead and then seal the bead to the glass envelopewall. In another type of envelope structure where the glass wall iscast, using powdered glass moulding techniques, the leads are sealeddirectly in place when the glass wall is cast.

In either case th likelihood of cracks developing at the seals is aproblem. The reason for these seal failures is not thoroughlyunderstood, there probably being several contributory causes such asvoltage gradients, electron bombardment and thermal conditions. There isalso evidence that electrolysis occurs in the glass which results in adeterioration of the glass and/or metallic oxide adjacent the seal; andit further appears that, with negative grid type tubes, the crackingproblem is worst at the grid seals which operate at a negative potentialwith respect to the anode.

A number of devices have been suggested for protecting the glass at theseals, some involving metal caps such as shown by the patent to Heintz,No. 1,970,750 and others involving protective metallic coatings such asdescribed in patents to Foulke, No. 1,832,009, and to Wagener, No.2,329,- 019. These devices are applied over the glass surfaces after theseals have been made, and although applied superficially, are more orless helpful in alleviating the cracking troubles.

The broad object of our invention is to profide a more effective anddependable solution to the seal cracking problem.

Another object is to provide a protected seal structure which willwithstand extremely adverse operating conditions without failure,whereby the tube will give better performance, operate at higherratings, and provide longer tube life.

Still another object is to provide a seal structure in which theprotective means is incorporated as an integral part of the glass at theseal.

Further objects include the provision of an improved seal of thecharacter described which is easy to make in mass production, and inwhich the parts of the seal are maintained visible for inspection.

The invention possesses other objects and features of advantage. some ofwhich, with the foregoing, will be set forth in the followingdescription of our invention. It is to be understood that we do notlimit ourselves to this disclosure of species of our invention, as wemay adopt variant embodiments thereof within the scope of the claims.

Referring to the drawings:

Figure l is a side elevational view of a glass envelope type of electrontube embodying our 1111-- proved seal structure at the grid lead.

Figure 2 is a vertical sectional view showing a bead on the grid leadprior to scaling to the envelope wall; and

Figure 3 is a similar view illustrating an alternate construction.

Figure 4 is a vertical sectional view of a modified type of envelope andseal structure. incorporating our improvements.

In terms of broad inclusion, an electron tube embodying our inventioncomprises an envelope having a glass wall, an electrode in the envelope,and a metal lead for the electrode extending through and sealed'to thewall, the glass surrounding the lead being impregnated with metallicpowder. In our preferred seal structure the metallic powder isconcentrated in the central regions of the glass and extends radially ofthe lead.

In greater detail, and referring to the drawings, Figure 1 illustratesour improved seal structure 2 incorporated at the grid lead 3 of aconventional triode type vacuum tube having a glass envelope 4. In thisall-glass style of tube envelope construction, the main body of theenvelope is usually made of a preformed glass blank. Other parts of thetube include an anode 6 supported on a lead I, a, grid 8 supported onthe lead 3, and a suitable cathode such as a filament support d on theleads 9. The leads for the various electrodes are preferably of arefractory metal such as tungsten, and the glass employed is selected tomatch the expansion characteristics of the metal as is common practice.

In our improved seal sllucture, as illustrated at the grid seal 2, theglass surrounding the lead 3 is impregnated with a metallic powder H toprovide a region about the lead in which the glass itself is metallized.The metal of the powder is preferably the same as that of the lead,namely, if a tungsten lead is used the impregnating powder is also oftungsten. The usual procedure of first beading the lead and then sealingthe bead to the envelope is followed in the practice of our invention.the metallic powder ll being incorporated in a flanged glass bead 12 asshown in Figure 2.

Bead i2 is preferably cast about lead 3, using powdered glass in agraphite mould which is heated in a suitable furnace to fuse the glass,the lead 3 being first oxidized to insure the desired glass-.to-metalseal. In one form of our invention the metallic particles l l areuniformly distributed throughout the bead, as shown in Figure 2. This isaccomplished by mixing the metal powder uniforml with the glass powder,which mixture is then placed in the mould and heated to melt the glass.The metal powder is preferably quite fine, and a sufiicient quantity isincorporated to provide an electrical conducting path in the glass. Theonly precaution to observe in this connection is to avoid adding toomuch metal powder, as an excess tends to make the glass porous and mightproduce an air leak through the envelope wall. After removal from themould the beaded lead is sealed to the tube envelope in the usualmanner, the impregnated bead thus providing the metallized glass wallportions surrounding the lead 3 shown in Figure 1.

The modified structure illustrated in Figure 3 is ordinarily preferredbecause it avoids any possibility of a porous wall. In this case themetallic powder ii is concentrated in a disk-like zone lying in thecentral regions of the glass and extending radially of the lead. Thisstructure is made by first placing a layer of glass powder in the bottomof the mold, then a layer comprising a mixture of glass and metalpowders, and finally another layer of glass powder. The mold is thenheated to the melting point of the glass which fuses the mass, includingthe central metallized glass zone, into a solid body. Since the outerglass layers insure vacuum tight surfaces, the central zone may havequite a high concentration of metal. For example, we have used for thecentral zone a mixture containing one part by \veightof tungsten powderto five parts by weight of glass powder with good results, the powderedglass employed being preferably finely divided to rovide a homogeneousmixture. As shown in Figure 2, the central metallized layer makescontact with the head 3. In either case a bead is provided which isvitreous in character yet provides an electrical conducting pathradiating outwardly from the lead within the glass wall. The flangedbead I2 is made suiiiciently large in diameter, say an inch or so for amedium size transmitting type tube, so that the internal E185;conducting path extends for a considerable distance out away from thelead 3.

We have operated tubes of the type shown in Figure l with our improvedseal structure alongside similar tubes of conventional sealconstruction, and found that the improved tubes did not develop gridseal cracks under conditions which caused the conventional seals tocrack at frequent intervals. We also found that tubes with our improvedmetallized glass seals would withstand adverse operating conditions suchas higher ambient temperatures, and would operate successfully whenexceeding the normal tube ratings, without seal failures.

While the precise causes of seal cracking are not fully understood wehave found that our metallized glass seals provide a practical solutionto the problem. It is believed that the provision of an electricalconducting path within the glass itself, which path radiates outwardlyfrom the lead within the glass body in the region of the seal, is theimportant feature contributing toward the improved results. Anotherdesirable attribute of our seal construction is that the protectivemeans is not applied as an external device butrather is incorporated asan integral part of the glass envelope. In our construction the glass atthe seal is visible for inspection at all times.

Figure 4 shows our improvements embodied in another type of tubeenvelope construction. In some tubes 2. part of the envelope may be ofmetal such as the ring-shaped piece II, which ring is brazed or sealedto another part of the envelope. When an electrode lead I4 is broughtout through such a ring it is common to cast the entire glass wallsection I in a. graphite mould. using powdered glass mouldingtechniques. In this case we form a metallized disk-shaped area I!surrounding the lead I, the procedure being to partially fill the mouldwith powdered glass. then pour in a mixture of metal and glass powdersto occupy the area l1. and then complete the filling of the mould withpowdered glass. After the mould has been fired to fuse the glass theentire unit, including the sealed-in lead ll and ring member i3, isready for further tube assembly upon removal from the mould.

We claim:

1. An electron tube comprising an envelope having a glass wall, anelectrode in the envelope. and a metal lead for the electrode extendingthrough and sealed to said wall, the glass surrounding the lead beingimpregnated with metallic powder, said impregnated glass extendingradially of the lead for an appreciable distance to provide a disk-likemetalized glass protective shield in the envelope wall surrounding thelead.

2. An electron tube comprising an envelope having a. glass wall, anelectrode in the envelope. and a metal lead for the electrode extendingthrough and sealed to said wall, the glass surrounding the lead beingimpregnated with metallic powder, said powder being concentrated in thecentral regions of the glass and extending radially from the lead for anappreciable distance to provide a disk-like metalized glass protectiveshield in the envelope wall surrounding the lead.

3. An electron tube comprising an envelope having a glass wall,electrodes in the envelope including a grid, 2. metal lead for the gridextending through and sealed to said wall, the glass surrounding thelead being impre nated with metallic powder, said impregnated glassextending radially of the lead for an appreciable distance to provide a.disk-like metalized glass protective shield in the envelope wallsurrounding the lead. 4

WILLIAM W. EITEL. MARTIN E. WOLFE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 335,160 Thomson Feb. 2. 1886485,418- Pollard Nov. 1, 1892

